The present invention generally relates to salts of complexes of aminocarboxylic and hydroxycarboxylic acids, and in particular, to the process of preparing salts of iron complexes of aminocarboxylic and hydroxycarboxylic acids, such
iron chelates have numerous applications, such as in agriculture, in photographic processing, and as food additives.
One conventional route to ferric ammonium EDTA complex is the reaction of diammonium EDTA solution with sponge iron at a temperature of 60.degree. C. or higher for approximately 10 hours, thereby producing ferrous EDTA. The ferrous EDTA is then oxidized to ferric EDTA by air sparging or many hours. However, the production of the ferrous EDTA intermediate releases H.sub.2, which is potentially explosive. The evolution of H.sub.2 also often causes a foaming problem which limits the batch size. The sponge iron contains traces of sulfur and phosphorus; during the reaction with EDTA these are emitted as phosphine (PH.sub.3) and hydrogen sulfide (H.sub.2 S) which generate obnoxious odors. In addition, the total processing time is in excess of 24 hours.
U.S. Pat. No. 4,558,145 to Smith et al discloses a process for preparing 5% iron solutions of the ferric chelate of hydroxyethlyenediaminetriacetic acid from the trisodium salt of that acid, nitric acid and metallic iron. The ferrous chelate thus produced is then converted to the ferric chelate by air oxidation.
Japanese Kokai 53-35929 discloses a process for preparing ferric chelates wherein a mixture of 5-74 weight % Fe.sub.3 O.sub.4, 25-95 weight % iron, and depending on the circumstances, 0-60 weight % of a water soluble iron salt are made to react with a chelating agent or its alkaline salts in an aqueous medium not above room temperature. The ferrous chelate is simultaneously or subsequently oxidized to ferric by air sparging. This process has the advantage that little hydrogen is evolved; however, all the iron complex produced from the Fe.sub.3 O.sub.4 /Fe.degree. reaction is ferrous iron and thus all must be oxidized to ferric iron.
Japanese Kokai 59-167595 discloses a process for Preparing ferric chelates wherein the rate of reaction of the chelating agent with hydrated iron oxide is accelerated by the addition of a reducing agent such as hydrazine or sodium hydrosulfite.
U.S. Pat. No. 3,767,689 to Donovan et. al. discloses a process for preparing water-soluble ammonium salts of ferric aminocarboxylic acid complexes by heating iron oxide with an aminocarboxylic acid or a partially neutralized aminocarboxylic acid in an aqueous medium, and neutralizing the resulting ferric complex by reacting it with a base, such as ammonium, sodium, or potassium hydroxide.
U.S. Pat. No. 4,364,871 to Svatek et. al. discloses a process for preparing aminopolycarboxylic acid chelates of iron by reacting ammonia and the aminopolycarboxylic acid in a mole ratio of about 1-1.5:1 (NH.sub.3 aminopolycarboxylic acid) in the presence of iron oxide. After the iron oxide is completely reacted with the chelant, the mixture is cooled and sufficient ammonia is introduced to dissolve and maintain the iron chelate in solution. The reaction mixture is cooled, and contacted with air to oxidize any remaining ferrous to ferric.
These processes suffer from various drawbacks, such as the necessity of elevated reaction temperatures which results in the deleterious decomposition of the aminopolycarboxylic acid, the necessity to use expensive synthetic iron oxides and/or reducing agents to achieve acceptable reaction temperatures and times, the generation of all iron in the Fe(II) oxidation state, or the evolution of explosive hydrogen.